Liquid crystalline Blue Phase (BP) is a special phase which is macroscopically between the isotropic phase and the cholesteric phase, and often occurs in high chiral liquid crystal systems. Microscopically, the blue phase is a lattice defect phase without exhibiting birefringence, and the magnitude of lattice parameters of BP is comparable the wavelength of visible light (i.e., hundreds of nanometers). The blue phase can be divided into three subphases according to their crystal structures, which are designated as blue phase I (BPI), blue phase II (BPII), and blue phase III (BPIII), respectively. The corresponding crystal lattice structures thereof are body-centered cubic structure, simple cubic structure, and amorphous state, respectively. The blue phase crystal material is considered as the most promising next-generation liquid crystal display material due to its microsecond level response time to an electric field. In comparison with a conventional liquid crystal display, a blue phase liquid crystal display has the following four prominent advantages.
(1) Due to the microsecond level response time to an electric field, the blue phase liquid crystal can be field-sequentially driven and thus a color filter is no longer required, which not only reduces the material cost, but also improves the efficiency of the backlight source.
(2) Due to the microscopically optical isotropy, the internal surface of the substrate of the blue phase liquid crystal display does not need orientation treatment, which can simplify greatly the manufacturing process thereby reducing manufacture costs.
(3) The blue phase liquid crystal display has a wide visual property. Thus, a visual compensation film is no longer required, and the width of the visual field can be controlled according to the practical requirements.
(4) The light transmissivity of the blue phase liquid crystal display is not affected by the substrate gaps, and thus the substrate gap does not need to be strictly controlled. Thus, the manufacture process can be substantially simplified so as to reduce the manufacture costs.
However, since the monomer content in a polymeric blue phase liquid crystal is usually less than 10 wt %, the polymeric network is very prone to deformation under damages caused by the electric field, which eventually reduces greatly the life of the liquid crystal display.
In order to improve the performances of the blue phase liquid crystal, it is possible to disperse uniformly the droplets of a blue phase liquid crystal in the polymer matrix to form a polymer dispersed blue phase liquid crystal. A polymer network exhibits a better stability in an electric field, and is convenient for a large-scale production.
However, because the polymer network has strong constraining effect on liquid crystal molecules, the blue phase liquid crystal has high driving voltage. Moreover, in a strong electric field, blue phase liquid crystal droplets are very prone to phase transition which leads to electro-optical hysteresis. Therefore, development of a blue phase liquid crystal composite material having low driving voltage and no electro-optical hysteresis property has important theoretical significance and practical value.